1. Field of the Invention
The present invention relates to a step motor controller for accelerating and decelerating a step motor so as to position the motor to the target stop position that changes constantly. More particularly, the invention relates to a step motor controller that reduces beat vibrations resulting from repeated acceleration and deceleration of the step motor connected thereto.
2. Description of the Related Art
Recently, there exist schemes whereby the throttle valves of automotive engines are controlled electronically using step motors. These step motors in the vehicle operate in conjunction with step motor controllers that are used extensively to control the actuation and stopping of the motors.
What is required is to control the opening of each of the throttle valves in keeping with the extent to which the accelerator is operated. The requirement is met by the step motor controller which first of all measures the amount of accelerator operation using a potentiometer or the like. The measured value of the potentiometer is sampled at predetermined intervals and converted from analog to digital format for use as the target value. The controller controls the stop position of the step motor in accordance with that target value, thereby controlling the opening of each of the throttle valves.
The difference between the step motor controller to control the stop position of the step motor for control of the throttle valve opening and ordinary step motor controllers is as follows. For one thing, the amount of accelerator operation varies constantly with the operating status of the vehicle. For another, the opening of the throttle valves needs to be controlled quickly in response to the extent to which the accelerator is operated. On the other hand, for ordinary step motor controllers, the target stop position of the step motor is not supposed to vary at short notice. Thus, with these controllers, what is important is how precisely to drive and stop the step motor to its target stop position in accordance with a predetermined speed pattern.
There have been growing needs for controllers by which to control electronically the amount of accelerator operation by use of the step motor. One such controller for controlling the step motor in opening and closing throttle valves is disclosed illustratively in Japanese Patent Laid-Open No. 138855/1986. The disclosed step motor controller involves comparing the target value designated by accelerator with the current value of the step motor and, if a difference exists between the two values, controlling the revolutions of the step motor in accordance with a previously stored table.
Conventional step motor controllers of the above kind have two major disadvantages.
(1) One major disadvantage is as follows. The step motor controller samples at predetermined intervals the measured value from the potentiometer measuring the amount of accelerator operation. The step motor is controlled so as to approach the target value thus sampled. When the target value becomes constant, the step motor is stopped. It is common practice to rotate the step motor up to the target value as quickly as possible by accelerating the motor at the start of its rotation and by decelerating it when the target value is approached. The process of repeatedly accelerating and decelerating the step motor tends to cause the step motor-driven system, including the throttle valves, to generate beat vibration. The vibration destabilizes the throttle valve positions and thus causes the flow rate of mixture intake into the engine to become unstable. As a result, the speed of the vehicle can fail to keep pace with the extent to which the accelerator is operated.
FIG. 6 is a view showing how a step motor is controlled illustratively by a conventional step motor controller, with the step motor generating vibration. In FIG. 6, reference character M represents those changes in the target value which are taken of the amount of accelerator operation by potentiometer, sampled as analog data at intervals of a predetermined sampling time A, and converted from analog to digital format. The sampling time A is generally about 6 milliseconds (abbreviated as ms hereinafter), determined in view of the tasks of the CPU that provides main vehicle control while also controlling various auxiliary devices configured.
Reference character P denotes the current step that the step motor is in. The current step P is, as shown in FIG. 6, changed step by step. MSPD indicates the condition of the driving pulses applied to the step motor. Specifically, the condition is given in terms of driving frequency and excitation time. The driving frequency and excitation time of each of the conditions given to the step motor are set forth illustratively in the table of FIG. 5. The condition made up of the two values is determined by the difference between the target value and the current position of the step motor.
When the value M varies during operation, a difference MA occurs between the target value and the current position of the step motor. The step motor is then accelerated from MSPD=1 to MSPD=2 to eliminate the difference MA. As the current position of the step motor approaches the target value, the step motor is decelerated from MSPD=2 to MSPD=1 so as to reduce the vibration that will occur upon motor stop.
According to the table of FIG. 5, the total time B during which the step motor is accelerated and decelerated in the above manner amounts to 6.858 ms, as shown in FIG. 6. This is because the excitation time at MSPD=1 is 2,000 .mu.s (2.000 ms) and that at MSPD=2 is 1,429 .mu.s (1.429 ms).
The opening of the throttle valves controlled by the above-described step motor controller is known to generate periodic beat vibration when measured, as indicated by reference character S in FIG. 6. The beat vibration of the throttle valves destabilizes their positions and thus causes the flow rate of mixture intake into the engine to become unstable. The result is that the speed of the vehicle can fail to keep pace with the extent to which the accelerator is operated.
The beat vibration of the step motor, if promoted appreciably, can lead to an out-of-step condition in which the step motor fails to follow the command pulses. This also makes it impossible for the vehicle to keep up in speed with the extent to which the accelerator is operated.
(2) The other major disadvantage of the conventional step motor controller is as follows. When the step motor controller stops the step motor, the step motor-driven system still vibrates for some time and thereby causes the flow rate of mixture intake into the engine to be unstable for a certain period of time. This also results in the failure of the vehicle to keep up in speed with the extent to which the accelerator is operated.
In order to avoid these deficiencies, the conventional step motor controller, illustratively in reversing the step motor, is required to set the throttle valves in the target stop position and keep them there for at least 20 to 30 ms before the reversal. This corrective measure necessarily entails a delay in keeping up with the changes in the extent to which the accelerator is operated.